1. Field of the Invention
The present invention relates to communication systems, and more particularly to a decision directed flicker noise cancellation apparatus and method for reducing flicker noise in communication devices including wireless communication devices configured in a zero intermediate frequency (ZIF) architecture.
2. Description of the Related Art
FIG. 1 shows plots of flicker noise for Orthogonal Frequency Division Multiplexing (OFDM) signals, Complementary Code Keying (CCK) signals, and Barker signals, respectively. As shown, flicker noise is spectrally coincident with OFDM, CCK and Barker signals. For OFDM, the four lower frequency tones on either side of DC are affected. CCK is most sensitive whereas Barker is a little less sensitive as having less signal energy at zero frequency.
The present disclosure concerns a novel system and method for reducing flicker noise (1/f), including wireless devices using any combination of Barker, CCK and OFDM type signals. Flicker noise is associated with the level of DC in a signal and has a 1/f spectral power density in which the noise is greatest near DC and rolls off with increasing frequency. In communication systems, flicker noise causes harmful effects in the receiver, and is generated within analog processing circuitry for converting a received signal to baseband (BB). Although flicker noise may be present in strong signals with a relatively high signal to noise ratio (SNR), flicker noise is particularly problematic for weaker signals in which it becomes more difficult to distinguish between signal and noise. In wireless communications, for example, flicker noise depreciates the quality of signal decisions resulting in relatively noisy constellation plots, which increases the packet error rate (PER) and reduces communication efficiency and performance.
Prior techniques sought to avoid the 1/f peak at zero frequency at a cost of increased frequency and sampling rate. Such techniques invariably resulted in increased power levels and higher cost. It is desirable, however, to reduce power consumption and cost while simultaneously increasing performance and communication efficiency. It is desired, for example, to reduce or eliminate the effects of flicker noise without modifying frequency or sampling rate.
The present invention is illustrated for use in the wireless local area network (WLAN) environment, particularly direct conversion or zero intermediate frequency (ZIF) configurations in which there is significant gain at low frequencies and DC. The present invention is equally applicable, however, in any type of system in which flicker noise is generated. In a ZIF WLAN configuration, flicker noise is typically generated in radio frequency (RF) analog circuitry and degrades digital signal processing in the digitized baseband. Furthermore, although flicker noise exists in bipolar devices, it is substantially greater in CMOS (Complementary Metal Oxide Semiconductor) devices when CMOS technology is employed for implementation. It is desired to combine analog and digital circuitry onto a single chip or integrated circuit (IC) employing CMOS technology. Therefore, it is desired to reduce or otherwise eliminate flicker noise to enhance the quality of signal decisions and improve communication efficiency.
Non-coherent flicker noise cancellation techniques have trouble distinguishing modulation noise from flicker noise. Non-coherent flicker noise mitigation techniques, such as the high pass filter (HPF), affect both the signal and the noise. Hence, the flicker noise estimate is corrupted. Furthermore, the HPF distorts (e.g., notches) the signal, degrading PER performance. It is desired to provide relatively simple, decision-directed joint DC and flicker noise cancellation.